Servo systems



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SERVO SYSTEMS Filed Feb. I 9,

nited tates Patent @i SERV() SYSTEMS George Orlo, Arthur Ernest Henry Elmer, and Richard Abdiel Chorley, Gloucester, England, assignors to British Messier Limited, Gloucester, England, a British company Application February 9, 1956, Serial No. 564,521

Claims priority, application Great Britain February 9, 1955 16 Claims. (Cl. 60--97) This invention relates to servo actuating systems and is concerned with those in which the actuating units are duplicated for safety reasons.

One of the difculties encountered in duplicated servo systems is that of detecting which one of the two actuating units is faulty and of providing means capable of preventing the faulty unit from interfering with the operation of the system by the other unit. The present invention provides an arrangement for overcoming this diiliculty.

According to the invention a servo actuating system comprises:

(a) Twin servo-units each having a movable output member, means for transmitting a duplicated control signal to the servo-units said control signal having a` characteristic which is variable in accordance with the position required to be taken up by the output members, with a control arrangement for each servo-unit including means associated with the output member to produce a follow-up signal indicating the actual position of said member, means for receiving and comparing the control and follow-up signals and means acting in response to the lack of correspondence revealed by said comparison to cause working medium to act in the servo-unit and move the output member in a sense to reduce the difference between the actual position of said member as signalled by the follow-up signal and the required position signalled by the control signal,

(b) Release means associated with each Servo-unit for rendering the output member at least of said servo-unit freely movable in both working directions irrespective of the value or sense of the aforesaid difference indicated by the signal comparison, operation of said release means being initiated by said difference when the latter reaches a predetermined abnormal value, and

(c) Operative connections between each of the output members and a part to be actuated, said operative connections permitting relative movement between the output members suicient for the aforesaid difference be tween the actual position of the output member of one servo-unit and the required position of the said member to build up to the predetermined abnormal value while the difference between the actual position of the output member of the other servo unit and the required position of the said member remains within normal limits.

In the operation of the servo system according to the invention equal control signals are supplied to the signalreceiving means o-f both units, but if one unit fails to respond to such signal while the other unit does so respond (this differential action being possible by virtue ot the relative movement permitted between the output members) the comparison of the control and follow-up signals will show a greater lack of correspondence in the faulty unit than in the sound one and this lack of correspondence will eventually reach a value which will actuate the release means associated with the faulty unit for rendering its output member freely movable in both directions. When this occurs the sound unit can continue to operate without interference from the faulty unit. Preferably in this condition the output members are locked together to eliminate the lost motion which would otherwise be present in the system.

In some applications it is possible to divide the part to be actuated into two sections, and in this case, according to a feature of the invention, each output member is connected to one of such sections and means are provided for locking the sections together, such means being actuated automatically to lock the sections together when one of the output members is rendered freely movable in both working directions.

Where it is not possible or convenient to divide the part to be actuated as above described, the output members may be connected to the said part by a dierential system, means being provided for limiting and/or preventing the differential action, said preventing means, if present, being actuated to lock the output members together when one of said members is rendered freely movable in both working directions.

According to a further feature of the invention a servo system as defined in the third, fifth or sixth paragraph of this specification may be combined with a remote control station comprising an operators control member and twin signal transmission means operable simultaneously by movement of the control member to transmit the duplicated control signal to the servo units.

Means may be provided whereby the control signal receiving means of the servo-units may be switched either to the signal transmitting means controlled by the operators control member or to signal transmitting means controlled by an automatic device.

In a system as defined in the last preceding paragraph means may be provided for moving the operators control member in accordance with movements of the part actuated when the system is under the control of said automatic device.

These and other features of the invention will be eX- plained more fully in the following description of a particular example in which the invention is applied to an electro-hydraulic servo system for a flying control of an aircraft. The system is shown diagrammatically in the accompanying drawings, in which:

Figure 1 is a general lay out, and

Figure 2 shows a form of relay device that can be used therein.

In the drawings (Figure l) two sections of an aircraft flying control surface to be actuated are shown at 1 and 1a respectively, and each section is connected to one half of a duplicated servo system. As the two halves of the servo system are identical it will be convenient in the description to refer to the corresponding parts by the same reference numeral but with suix a for the half which is lowermost in the drawing. The same description will also apply to both halves.

Considering the uppermost half of the system, the control surface 1 is connected by suitable linkage 2 to the output member 3 of a servo-unit indicated generally by the reference 4l. The section 1 of the control surface carries a hydraulic cylinder 5 containing a piston 6 from which a locking bolt 7 projects to engage a recess 8a in the section 1a of the control surface when the piston is moved downwardly by a spring 9. By admitting liquid under pressure through a pipe l0 into the lower chamber of the cylinder the locking bolt is withdrawn so that the control surface sections 1 and 1a are free to move relatively to one another.

locking bolts of tapered formation it can be arranged that the two sections of the control surface are brought into alignment with one another when the bolt is engaged.

Such relative freedom need only extend through a small range and by making the l Inthis-.examplethezseryo.. unit comprises a hydraulic jack.. ll,.the` rarnof-which,,constitutes theroutput mem; ber 3, an electro-hydraulic control Valve 12, an electrical amplifier 13 and an electrical followeup transmitter 14 operated. by; the output member 3. The;amplier 131er ceives zaxr: electrical controlzzsignal '.through; a line; 1.5 and y an electrical .follow-up signal, through ailiner'lvand; ampliliedvsignals are transmitted throughflinesflf/ and 18i'to the valve 12. This valvecomprises.-ahydraulic-controlz elementmoved bythe diierentialaction of twozelectri cal windingsof. equal power (not shown) supplied: by the lines 17. and 18, the line.49 vbeing. a. commonconductor carrying currentzequalfto the sumgor. difference, ofthe currentsin .the .lines .17 and 13.` It willgbeunder.- stood therefore that the. hydraulic control. element. of. the valve` 12 -is.moved.in.dire.ction.and magnitude corresponding to the difference betweenithecontrol signal and..

the` follow-up. signal. lfpit is desired toV work;with controlv andv follow-up signalsof different power, the powery of the windings can be, adjusted to compensate for` this-,-

nals permits an increase'of pressure to be developed oni that side of the jack piston necessary tomovethe. pistonv n the direction bringing the follow-up.v signal vtoward equality with the control signal. The arrangement thus operatesin a well known manner to maintain the position of the output member 3 in conformity with thei con@Y trol signal supplied through the line 15.

The two servo units 4 and 4a aresupplied with equal control signals either from an automatic pilot A, when a.- selector switch B is set to connect thesignal-receiving lines 15 and 15a respectively to switch terminals 23, 23aV receiving the output of the automatic pilot, or from twin` transmitters 24, 24a controlled by'a'manuallyfoperated input member C, when theswitch B is setto connect lines 15v and 15a with contactsZS, 25aconnected by lines 26, 26a with the transmittersr24, 24a. Itlwill be clear that `the transmitters 24, 24a-fand. the automatic pilot A may be locatedin any.convenient-position,-pos.A

sibly remote from the servo units 4 and 4a.

The pipe--from the lock? cylinder Sis-connected to the pipe 20 from the pump 19 so that in the event of loss ot`=hydraulicpressure in thepipe 20, `which would renderv the'nservo-unitE 4f4 inoperative, the locking bolt 7 engagestherecessa and'locks lthe two partsof thereontrol surfacef together. At thesame time a pressureoperated by-passV rvalve: r64 opens communication between opposite endsi of thejack cylinder'll so that the output memberfS'is free to move inboth' working directions. Thus both parts 1 and1'a of the= control surface can continue to be operated by the-servo-unit 4a.`

To'detect a fault in the lservo unit 4 itself, which fault prevents -displacement of the'outputmember 3 in proper response torthecontrol signal=supplied to the'line 15, a relay: 27 isf'provided having windingsfZS and 29 differentially-energised from the lines-17 and 18 so as to closecontacts v30 whenthe diie'rence between the-controlfsgnal carried-f bythe-:line- 17' and the follow-up signalcarriedi by the= line-lsf-exceeds -the greatest steady difference oceurringfduring;.normal working of the system. Since harmless:transient*signalzdi'erences; of higher intensity mayjroccurfduring#normalworkng, the relay 27 lpreferablyfincludes a delay-action'device'- adequate to prevent closing oflth'e Ycontactsstllbysuch transients.' Thedelayactionffdeviceshould'-be of Fa; kindv in which timing ystarts on.-teaching:V ay critical 'il level 1 off energisationjyand ywhich the windings. as the; maximum. permissible difference. be

has a rapid re-setting action when the energisation falls below this. level..v T'necontacts, once-closed,.should.also remain closed until reset oy hand after the fault in the servo-unit has been put right.

A delay action device with these characteristics can be constructed as follows (Figure 2:);

The differential windings .28. and .29v surround a common armature rod 52, whichrodmoves axially endwise through tween;L the;-signals in;..the two windings is. reached. If said'. difference ismaintained, one endgof the rod eventually reaches and closes the relay contacts 30, but to avoid closing` of thecontacts under theeiectof the. aforesaid transient signal differences the rod is retarded in its travel by a dashpotdevice 53.' The` dashpot consi-sts'ofia piston S4 on the opposite end-of therod 52 working in an oil lled cylinder 55, oil being transferred from one cylinder chamber to the other through a one-way restrictor 56 in 'the piston when the ,armature rod moves. A return spring 57 is provided inside the cylinderb`earing on the piston y54,V

to urge the'armature rod 52 awayfrom the relay contacts 30. When the armature rod under the control of the windings 28, 29 has moved far enough against theaction. of thereturn spring 57"to close the relay contacts, ,a .springl loaded. latch 58 'engages in arecess 59 in theside of the rod"52 and thereby prevents it from returninga-nd allowing. therelay contacts toopen although the energising current'.

to` therelay windings 2S, 29 be varied or cut'oif The latch has a tail portion 60 forming a further armaturerod surroundedby an energising coily 61, so that the latch..

can be withdrawn at will by operation of anfelectrical control, after the fault inthe system has been rectified.VV kThis device is only one method of. providing a delayed action relay, and any -other suitable devicenot necessarily hydraulic, which will servethe same purpose maybe used instead.

Closing of the contacts 30' causes energisation of. the

winding 31 of an electromagneticallyoperated off-loading valve 32 which comprises a normally open valve member.' 33in circuit between the pump 19 and the pressure supply.

pipe 20'and a normally closed valve member '34 in circuit between the pressure supply, pipe and they reservoir 21. Energisation of the windingl reverses thesevalve.Y members so that the pressure supply. pipe 20 is. cut ot.y from the pump 19 and. connected. to the. reservoir 21.. The pressure in the' pipe 20 consequentlyfallsto thatofv the reservoir 21 so that, in` the manner already described,

the bolt'7s projected to lockthe partsl and 1a,.of the. control, surf ace together, and. the. pressure-operated byY-,.v pass valve opens. communicationbetween opposite ends.. of thejackcylinder 11. Both partsoithecontrol surface.

Y can then continue to be operated. by .the.1servo.-unit 4a.

Energisation of the winding 31.0r of anotherrelaydevicey in parallel .with it vmay also be arranged .to disconnect .the

electrical Power supply from .the faulty halfof thes'ystem.. In the specification accompanyingpUnited .States appli-- cation SerialNo. 557,17.4led 1anuary131956f there described a servo system controlled-alternatively by manual and automatic,l means ,-in which means are-provdedfor moving themanual control memberin responsetomove;

ments of the actuated partwhen the .system .isunderaut matic control. y An arrangemenLofthis.kind. is preferablyv included' in. the. present system-1,5, and as,- shownin. the..

drawingsr (Figure` 1), comprises va-.signal VtrausmitterBS operated'by the output member3..and..connected.`by, a..

line 36 to Va servo-unitk 37 which is. similar in .all respectsr to the servo-unit 4 except that it needonly be of sucient.v power to move themanual input. member, C, to whichjts output member 38.V is connected through a spring..39, The unit 3 7 thus includesa follow-up;transmitter,40.` .Dpr-- erated by the output member 38,. an .amplifier 41, .ajacle, cylinder 42 with a .bypass valve 65, and anl electro' hydrauliccontrol valve 43. To detect faults .in the servo.v

unit 37 'itis'als'o provided with a relay 44 similar togthej# relay 27. Closing ofthel contacts"45 lof` the relay-'44A is' arranged to energise the winding 31 of the off-loading valve 32 so that the whole of the upper most half of the duplicated system is put out of operation as already described. If desired, however, separate oli-loading valves may be associated with the servo units 4 and 37 so that only that servo unit which is faulty is put out of operation.

To enable the operation of the system to be checked prior to flight, switches 46 and 47 are provided in the follow-up signal lines of the servo-units. When either of these switches is opened and the manual input member C is moved, the control signal will not be progressively opposed by the follow-up signal, so that the relay 27 or 44, according to whether the switch 46 or 47 has been opened, will be energised to release the pressure in the supply pipe 20, as indicated by the reading of a pressure gauge 48. The parts 1 and 1a of the control surface should then become locked together, and this can be checked by observing that they follow the movements of the input member together.

In the system so far described it has been assumed that the cylinders 11 and 42 of the servo-units 4 and 3".' are permanently fixed in relation to supporting structure. However, by attaching these parts to the xed structure in a releasable manner, controlled by the fault-detecting relays 27 and 44, a further measure of safety against mechanical seizure of the jacks can be obtained. A suitable anchoring device for this purpose is described in the specification accompanying British patent application No. 33,479/54. This specification also describes an alternative construction embodying mechanical transmission members and mechanically operated control valves for the jacks whereby the basic principle of the present invention may be carried out.

We claim:

1. A servo actuating system comprising: (a) twin servo umts each having a movable output member, means for transmitting a duplicated control signal to the servo units sa1d control signal having a characteristic which is variable in accordance with the position required to be taken up by the output members, with a control arrangement for each servo unit including means associated with the output member to produce a follow-up signal indicating the actual position of said member, means for receiving and comparing the control and follow-up signals and means acting in response to the lack of correspondence revealed by said comparison to cause working medium to act in the servo unit and move the output member in a senseto reduce the difference between the actual position of sa1d member as signalled by the follow-up signal and the required position signalled by the control signal, (b) release means associated with each servo unit for rendering the output member at least of said servo unit freely movable in both working directions irrespective of the value or sense of the aforesaid diierence indicated by the signal comparison, operation of said release means be- 1ng initiated by said difference when the latter reaches a predetermined abnormal value, and (c) operative connections between each of the output members and a part to be actuated, said operative connections permitting relative movement between the output members suicient for the aforesaid difference between the actual position of the output member of one servo unit and the required position of the said member to build up to the predetermined abnormal value while the difference between the actual position of the output member of the other servo unit and the required position of the said member remains within normal limits.

2. A system as claimed in claim l, wherein means are provided for locking the output members together to move as one when one of the output members is rendered freely movable by its associated release means.

3. A system as claimed in claim l, wherein the part to be actuated is in two independently movable sections, one operatively connected to one of the output members 6 and the other to the other output member, and means are provided for locking the sections together to move as one said means being actuated automatically when one of the output members is rendered freely movable by its associated release means. v

4. A system as claimed in claim l wherein the servo units comprise fluid-pressure-actuated piston-and-cylinder units or jacks and the release means yof each acts to cut off the lluid pressure to the jack, and to allow free passage between one side of the jack piston and the other.

5. A system as claimed in claim 4, wherein each jack has an electrically-operated control valve for controlling the admission and exhaust of pressure lluid, the control and follow-up signals being electrical and employed (after amplification if necessary) to actuate said valve, and the release means comprises an electrically-operated release valve serving to disconnect pressure luid from the control valve and to connect it with exhaust.

6. A system as claimed in claim 5 wherein a differential electrical relay is provided to control the supply of operating current to the release valve, and lines feeding the control and follow-up signals to the jack control valve serve to energize said relay.

7. A system as claimed in claim l, including a remote control station comprising an operators control member and twin signal transmission means operable simultaneously by movement of the control member to transmit the duplicated control signal to the servo units.

S. A system as claimed in claim 7, including an automatic device (such as an automatic pilot) capable of transmitting the duplicated control signal, and switch means operable to connect the servo units for control by the automatic device instead of by the operators control member.

9. A system as claimed in claim 8 wherein means are provided for automatically moving the operators control member in accordance with the movements of the part to be actuated When the automatic device is in use.

l0. A system as claimed in claim 9, wherein the means for automatically moving the operators control member comprise twin auxiliary servo-units at the remote control station, one corresponding to each of the aforesaid main servo units and each having a movable output member operatively connected to said control member, means for transmitting to each auxiliary servo unit a signal indicating the position of the output member of the corresponding main servo unit, and a control arrangement of each auxiliary servo unit including means associated with the output member thereof to produce a follow-up signal indieating the actual position of said member, means for receiving and comparing said follow-up signal and the signal from the output member of the main servo unit, and means acting in response to the lack of correspondence revealed by said comparison to cause working medium to act in the auxiliary servo unit and move its output member in a sense to reduce any difference between the actual position of said member, as signalled by its followup signal and the required position that corresponds to the instant actual position of the output member of the corresponding main servo unit as signalled from said main servo unit.

ll. A system as claimed in claim l0, including release means associated with each auxiliary servo unit for rendering at least the output member thereof freely movable in both working directions irrespective of the value or sense of the positional difference indicated by the comparison of the follow-up signal of said member and the signal from the corresponding main servo unit, operation of said release means being initiated by said difference when the latter reaches a predetermined abnormal value, and wherein the operative connections between the output members of the auxiliary servo units and the operators control member are such as to permit relative movement between said output members sutlicient for the aforesaid member. of one auxiliary servo unit while the difference between theV actualposition of the. output member4 of the other auxiliary'servo unit andthe requiredposition ofthe.

said member remains within normal limits.

12.. A system as claimed in claim l1, wherein the auxiliary servo units comprise uid-pressure-actuated pistonand-cylinder unitsior jacksand the release means of each acts to cutoff the uid pressureto the jack, and to allow free passage between one side of the jack piston and the other.

13. Asystem asclaimed in claim l2 wherein. the jack of each auxiliaryV servo unit hasan electrically-operated control valve for controllingI the admission and exhaust ofpressure uid, the two signals received for comparison in the control. arrangement of. said servo unit being electrical and employed (after amplification if necessary) to actuate said valve, andthe release means for the unit comprises an electrically-operated. release valve serving to disconnect pressure fluid from said control valve and to connect it with exhaust..

14. A system as claimed in claim 13, wherein a dilerential electrical relay is provided to control the supply of operating current to the-release. valve of the auxiliary servo unit, and linesy feeding. the electrical signals tothe jack control valve of said unit serve to energize'said relay.

l5. A system as claimed in claim l, wherein for the purpose of testing,manually-operablermeans are provided forr interrupting each, of. thetransmissions conveying; the" follow-upl signals -of the. servoiunits.

1,6. A.. system as; claimed in claimT 3., wherein theservo units comprisef uid-pressure actuated pistonand-cylinder units or jacksf andthe Arelease means. of eachactsto cut: oithe uid-pressure Ito thejack, andto =allowvfreezpassager betweenone .side of. thel jack pistonr` and the other, .and

the. locking means for thel sections` of. the.V part tohel actuated comprisetwinrlocking `members'rboth spring=urged toward engagementbutheld out of# engagement normally by uid. pressure; acting' independently-'on twin lock-control pistons .oneior eachlocking member, the fluid pressures.

supply/,for one.lock control piston being taken from .the

supplyA to. one jack andfthat for the other.I piston from-- the supply. totheotherjack so thatf whenever one of the:

jacks has its fluid pressuresupply cut Gif-bythe associated. release means or, due to some failure: one or other` of the lockingmembers becomes'Y engaged;

References vCited in the tile of .thistpatent UNITED 'STATES PATENT 

